[R] linear trend line and a quadratic trend line.

[Peter Dalgaard]

Eric Fail wrote:
> 
> 
> Dear list users
> 
> 
> 
> How is it possible to visualise both a linear trend line and a quadratic trend line on a plot
> of two variables?
> 
> 
> 
> Here my almost working exsample.
> 
> 
> 
> data(Duncan)
> 
> attach(Duncan)
> 
> plot(prestige ~ income)
> 
> abline(lm(prestige ~ income), col=2, lwd=2)
> 
> 
> 
> Now I would like to add yet another trend line, but this time a quadratic one. So I have two
> trend lines. One linear trend line and a quadratic trend line. A trend line as if I had taken
> I(income^2) into the equation.
> 
> 
> 
> I know I can make two models and compare them using anova, but for pedagogical resons I wold
> like to visualise it. I know the trick from my past as an SPSS user, but I would like to do
> this in R as well. Se it in SPSS
> http://www.childrens-mercy.org/stats/weblog2006/QuadraticRegression.asp

There's no precooked function that I am aware of, but the generic way is 
like

rg <- range(income)
N <- 200
x <- seq(rg[1], rg[2],, N)
pred <- predict(lm(prestige~ income+I(income^2)),
   newdata=data.frame(income=x))
lines(x, pred)

as usual, "like" means that if you can't be bothered with making your 
example reproducible, I can't be bothered with testing the code!


Well, actually, I found the Duncan data in library(car), so I did in 
fact test...

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